1. Field of the Invention
The present invention relates generally to food product enclosures which have humidity and temperature control. In the most preferred embodiment, the invention relates to a humidity controlled food warmer wherein the amount of humidity added by a humidity control system is varied depending upon temperature deviations, more particularly upon a sensed relatively large deviation in temperature indicating that, for example, the enclosure has been opened. In the most preferred form of the invention, the controller is adapted to add humidity to the enclosure for a period of time related to the period of time the temperature of the enclosure deviates from a preset temperature by a predetermined amount.
2. Description of Prior Systems For Heating An Enclosure and Providing Humidity Control
Several patents have been issued on food treatment/holding devices which include humidity control. One such system is disclosed in U.S. Pat. No. 4,891,498 issued Jan. 2, 1990 to Fortmann, et al. and entitled "Food Treatment Cabinet With Flash Steamer". In this device, a water storage compartment includes a heater and means for delivering a plurality of pulses of water to a heater surface where the water is boiled into an evaporative state. The number of water pulses directed onto the surface is determined by the length of time that a cabinet door is opened. A limit switch in the door is connected to the control circuit to indicate the length of time, and upon sensing of the door being opened, a control circuit is reset resulting in a pulse of water being immediately applied to the heater surface. Much of the patent specification is devoted to a description of the control which includes a cube timer which operates solenoid valves for a preselected time period at preselected intervals. In a first interval, the pulses are added to provide initial moisture to the cabinet. In a second mode of operation, maintenance pulses are used while the door remains closed and in a third mode of operation, the system adds additional moisture, predetermined by a timer which measures the length of time that the door has remained open. In all cases, water is vaporized upon hitting the surface of the heater.
A parent to the aforementioned '498 patent is U.S. Pat. No. 4,835,368 issued May 30, 1989 to Fortmann, et al. and entitled "Food Treatment Cabinet With Flash Steamer". This device adds water to a heater surface capable of vaporizing the liquid as an incidence of the door being opened or closed. In short, this more basic patent utilizes the same humidity control addition system, but without the sophistication of the timer arrangement more specifically set forth in the later patent.
A "Microprocessor Controlled Food Treatment Cabinet With Flash Steamer And Compensating Humidity Control Routines" is described in U.S. Pat. No. 5,025,132 issued Jun. 18, 1991 to Fortmann, et al. This device, like the other Fortmann, et al. devices, uses a heating plate and a solenoid for delivering a plurality of pulses of water against the heater surface depending upon the amount of time that a cabinet door is opened. The operating parameters are controlled in the memory of a microprocessor, with separate and independent control logic for an initial pre-heat mode, as well as for normal operating modes. The patent primarily relates to the use of solid-state equipment to improve upon the relay technology used in the earlier system.
"An Environmental Chamber" is described in Truhan U.S. Pat. No. 3,424,231 issued Jan. 28, 1969. The fluid sump in this device includes heating and cooling coils, and control is provided at the sump for suitable actuation of the coils. Humidity is added to the cabinet by means of a water spray injected into a gas stream to saturate the gas with water at a controlled temperature. A fan then directs the gas through a gas treating chamber into the food containing cabinet. The patent does not disclose techniques for sensing the need for additional humidity.
Other patents involving food-holding chambers with humidity addition include U.S. Pat. No. 4,010,349 issued Mar. 1, 1977 to Lee for "Proofing Cabinet". This patent is directed to an immersion heated water tray in a system which also includes a vertical heater for inducing circulatory flow within the cabinet.
In Roderick, U.S. Pat. No. 4,039,776 issued Aug. 2, 1977 for "Closed Passage Heat Holding Apparatus", the focus is on the circulation system for creating a desired pattern within the cabinet and does not include any system for sensing when changes in the amount of humidity are required.
Another proofing cabinet is described in Allen U.S. Pat. No. 4,373,430 issued Feb. 15, 1983 in which the invention is directed to a self-cleaning mode of operation. The humidity control in this device includes a shallow water pan with a heater capable of vigorously boiling the water.
Another device which discloses control of water addition and temperature is U.S. Pat. No. 4,623,780 issued Nov. 18, 1986 to Shelton for "Collectramatic Food Warmer." Humidity is controlled by controlling the water temperature or by passing moist air over a second heater. This patent also does not include any device for sensing the need for increased humidity when the cabinet is opened.
A "Food Warmer Cabinet Control" is shown in U.S. Pat. No. 4,722,268 issued Feb. 2, 1988 to Rightley. Selective adjustment of temperature and vapor pressure is provided by using a reservoir containing a first heater to provide heat of vaporization and a second heater to heat the air in a chamber. The control means includes regulating the time periods for which energy is supplied to the first and second heaters. The control also provides means for automatically changing the temperature of the first and second heater during at least one occasion in the processing of the food to provide a sequence of different stages of air and water temperature.
A "Method And Apparatus For Water Vapor Control In Convection Ovens" is described in U.S. Pat. No. 4,876,426 issued Oct. 24, 1989 to Smith. Humidity is controlled by spraying jets of hot air over the water surface and, in an alternate embodiment, air is heated to one temperature while water is heated to another temperature and controlled using a thermostat.
Yet another cabinet for controlling humidity is described in U.S. Pat. No. 4,924,072 issued May 8, 1990 to Oslin for "Humidity Control For Oven Chamber". Differences between air and water temperature are sensed in the forced-air convection oven systems or the device can be used as a steamer, depending on control provided by the operator. Wet and dry bulb thermometers in the circulating gas permit the control of relative humidity, whether the oven is used to proof dough or hold cooked food at a relatively low temperature.
Other devices which include a water container and a control for changing humidity and/or in which humidity is sensed by a humidity sensor include U.S. Pat. No. 2,499,525 issued Mar. 7, 1950 to Person for "Dough Raising Box"; U.S. Pat. No. 4,426,923 issued Jan. 24, 1984 to Ohata for "Storage Device For Processed Foods"; U.S. Pat. No. 4,483,243 issued Nov. 20, 1984 to Cote for "Apparatus For Rising Dough"; U.S. Pat. No. 4,939,987 issued Jul. 10, 1990 to Smith for "Humidity Control System"; and U.S. Pat. No. 4,674,402 issued Jul. 23, 1987 to Raufeisen for "Apparatus For Thawing, Retarding And Proofing Bakery Goods".
While all of the foregoing patents mention, in part, certain features of the present invention, they all suffer one or more drawbacks. For example, using a simple time measurement to control humidity or temperature is not suitable in all instances, as ambient conditions can have a dramatic impact on what occurs inside a cabinet when the door thereto is opened. In addition, the amount of door opening may be a significant factor. Other systems which employ simple measurements of humidity and temperature, whether or not interrelated, do not provide the type of control and limits needed for particular containment applications. Accordingly, it would be advantageous to provide a system which maintains the desired humidity level, for a given temperature, and which does so in a controlled manner.